Chemically dependent adolescents constitute the most seriously disturbed end of the adolescent drug abuse spectrum, and are those at the greatest risk of death, disability and social dysfunction. New initiates into intravenous drug use, a high risk group for AIDS infection, are likely to be drawn from chemically dependent populations. Despite the severity of this problem, little is known of the importance of physical and psychiatric comorbidity or of psychosocial risk factors in the evolution of this severe degree of substance abuse, and less is known of its natural history. To address these questions, we propose a three year longitudinal study of 500, 15-18 year old chemically dependent adolescents receiving treatment in seven residential treatment centers under the aegis of the Massachusetts Department of Public Health. The aims of the study are to ascertain the prevalence of comorbidity among chemically dependent adolescent; to assess whether chemical dependence preceded or followed other conditions; to describe the social, psychological and familial features of chemically dependent adolescents; to assess the clinical course of subgroups of adolescents differentiated by psychiatric and physical comorbidity; and to evaluate a possible association of social and familial factors with clinical course. Psychiatric comorbidity (DIS) adapted to include information on Attention Deficit Disorders, and other conditions of childhood. Physical comorbidity will be ascertain by record review, and medical history. Clinical course will be ascertained by record review, and medical history. Clinical course will be assessed bimonthly and at six months after treatment termination by urine drug screens, staff assessments, patient self ratings, and measures of social functioning from the D.I.S. and the Addiction Severity Index eliciting information on interpersonal functioning, educational and occupational status, legal involvements, and problem behaviors.